Many highly differentiated in vitro epidermal and dermal/epidermal skin models are being developed which would be extremely useful for: 1) transdermal drug delivery systems, 2) skin transplantation, and 3) dermal toxicity testing. However, none of the current models accurately reproduced the barrier properties of human skin. In addition to invalidating barrier studies, this shortcoming is significant since it indicates that the basic keratinocyte cellular metabolism and physiology in vitro is abnormal. Utilizing a serum free keratinocyte culture system, Phase I research will systematically investigate the effect of various media additives chosen specifically to optimize the formation of barrier lipids and epidermal ultrastructure known to be important in the skin barrier function. Phase I experimentation, which will include structural evaluation at the light and electron microscopy levels, lipid analysis, and transepidermal permeability measurements, will demonstrate the utility of the proposed system. These studies will lay the groundwork for Phase II research which will be an in-depth analysis of the nutritional requirements needed to generate skin in vitro with the barrier and function of native skin.